If you receive money without verification and return it no questions asked, then you are opening yourself up as an avenue by which people can launder money. Every receipt and retransfer of that money legitimises that money further and makes you party to the crime. The money is in limbo and can be returned as soon as the regulatory needs are met, but to do otherwise just voids the whole process.
At this point they are depriving themselves of the money by refusing to verify themselves, it’s a basic identify and address check. This is an investment company, they weren’t sending us their rent money, they wanted to invest it. They were just pissed that we expected them to follow the same process that every client needs to follow when investing.